He defeated the eastern usurpers Macrianus Major and Lucius Mussius Aemilianus in 261–262 but failed to stop the formation of the breakaway Gallic Empire under general Postumus.

[3] He was the son of Emperor Valerian and Mariniana, who may have been of senatorial rank, possibly the daughter of Egnatius Victor Marinianus, and his brother was Valerianus Minor.

She was the mother of three princes: Valerian II, who died in 258; Saloninus, who was named co-emperor but was murdered in 260 by the army of general Postumus; and Marinianus, who was killed in 268, shortly after his father was assassinated.

Valerian left for the East to stem the Persian threat, and Gallienus remained in Italy to repel the Germanic tribes on the Rhine and Danube.

Division of the empire had become necessary due to its sheer size and the numerous threats it faced, and it facilitated negotiations with enemies who demanded to communicate directly with the emperor.

According to Eutropius and Aurelius Victor, he was particularly energetic and successful in preventing invaders from attacking the German provinces and Gaul, despite the weakness caused by Valerian's march on Italy against Aemilianus in 253.

[11] Sometime between 258 and 260 (the exact date is unclear), while Valerian was distracted with the ongoing invasion of Shapur I in the East, and Gallienus was preoccupied with his problems in the West, Ingenuus, governor of at least one of the Pannonian provinces,[12] took advantage and declared himself emperor.

He then hastily crossed the Balkans, taking with him the new cavalry corps (comitatus) under the command of Aureolus[15] and defeated Ingenuus at Mursa[16] or Sirmium.

[18] A major invasion by the Alemanni and other Germanic tribes occurred between 258 and 260 (it is hard to fix the precise date of these events),[19] probably due to the vacuum left by the withdrawal of troops supporting Gallienus in the campaign against Ingenuus.

Franks broke through the lower Rhine, invading Gaul, some reaching as far as southern Spain, sacking Tarraco (modern Tarragona).

[20] After devastating Germania Superior and Raetia (parts of southern France and Switzerland), they entered Italy, the first invasion of the Italian peninsula, aside from its most remote northern regions, since Hannibal 500 years before.

When invaders reached the outskirts of Rome, they were repelled by an improvised army assembled by the Senate, consisting of local troops (probably praetorian guards) and the strongest of the civilian population.

[22] On their retreat through northern Italy, they were intercepted and defeated in the battle of Mediolanum (near present-day Milan) by Gallienus's army, which had advanced from Gaul, or from the Balkans after dealing with the Franks.

The two Macriani left Quietus, Ballista, and, presumably, Odenathus to deal with the Persians while they invaded Europe with an army of 30,000 men, according to the Historia Augusta.

He sent his successful commander Aureolus against the rebels, however, and the decisive battle was fought in the spring or early summer of 261, most likely in Illyricum, although Zonaras locates it in Pannonia.

[34] In the aftermath of the battle, the rebellion of Postumus had already started, so Gallienus had no time to deal with the rest of the usurpers, namely Balista and Quietus.

Under his command, they besieged Cologne, and after some weeks the defenders of the city opened the gates and handed Saloninus and Silvanus to Postumus, who had them killed.

In 262, the mint in Alexandria started to again issue coins for Gallienus, demonstrating that Egypt had returned to his control after suppressing the revolt of the Macriani.

The correspondence of bishop Dionysius of Alexandria provides a commentary on the background of invasion, civil war, plague, and famine that characterized this age.

[44] Knowing he could not afford to lose control of the vital Egyptian granaries, Gallienus sent his general Theodotus against Aemilianus, probably by a naval expedition.

It seems that, at first, a major naval expedition was led by the Heruli starting from north of the Black Sea and leading to the ravaging of many cities of Greece (among them, Athens and Sparta).

According to some historians, he was the leader of the army who won the great Battle of Naissus, while the majority believes that the victory must be attributed to his successor, Claudius II.

[46] In 268, at some time before or soon after the battle of Naissus, the authority of Gallienus was challenged by Aureolus, commander of the cavalry stationed in Mediolanum (Milan), who was supposed to keep an eye on Postumus.

According to Aurelius Victor and Zonaras, on hearing the news that Gallienus was dead, the Senate in Rome ordered the execution of his family (including his brother Valerianus and son Marinianus) and their supporters, just before receiving a message from Claudius to spare their lives and deify his predecessor.

[54] The biased and largely fictional[55] account of that reign in the Historia Augusta describes him as a lover of luxury, who dressed in purple, sprinkled gold dust in his hair, and built castles of apples.

In Southern's view, these reforms and the decline in senatorial influence not only helped Aurelian to salvage the Empire, but they also make Gallienus one of the emperors most responsible for the creation of the Dominate, along with Septimius Severus, Diocletian, and Constantine I.

[62] The capture of Valerian in the year 259 forced Gallienus to issue the first official declaration of tolerance with regard to the Christians, restoring their places of worship and cemeteries, therefore implying a recognition of the property of the Church.